When using listsinceblock with a target_confirmations greater than the current block count, it returns "0000000000000000000000000000000000000000000000000000000000000000" as lastblock, which makes the method fail when using that value as blockhash

Expected behavior

The method shouldn't throw an error for a return value that is supposed "to feed back into listsinceblock the next time you call it"

Actual behavior

RPC returns error code -5, "Block not found"

To reproduce
On a pristine node where "blocks" is currently at 0, call listsinceblock '' 6, it will return "0000000000000000000000000000000000000000000000000000000000000000" as lastblock, and on next call (listsinceblock "0000000000000000000000000000000000000000000000000000000000000000" 6 it will throw an error

System information

It can be reliable reproducible without any extra steps

Proposed solution
Returning the genesis block hash when target_confirmations exceeds the current block count

Note: This is particularly useful on regtest, as on mainnet you would need to call with "640645" (as of now) to get the described issue, but on a pristine regtest even using a value as low as 2 makes the returned value unsuitable for next calls.